Optical element, lithographic apparatus including such optical element and device manufacturing method, and device manufactured thereby

ABSTRACT

An optical element includes a top layer which is transmissive for EUV radiation with wavelength in the range of 5-20 nm, and a structure of the top layer is a structure having an rms roughness value equal to or larger than λ/10 for spatial periods equal to or smaller than λ/2. The structure promotes transmission through the top layer to the optical element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/981,736, filed Nov. 5, 2004, which claims priority toEuropean Patent Application 03078495.3, filed Nov. 6, 2003, the contentsof both of which are herein incorporated by reference in theirentireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical element, a lithographicapparatus including such optical element, a device manufacturing methodand a device manufactured thereby.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning device, such as a mask, may be used togenerate a circuit pattern corresponding to an individual layer of theIC, and this pattern can be imaged onto a target portion (e.g. includingpart of one or several dies) on a substrate (e.g. a silicon wafer) thathas a layer of radiation-sensitive material (resist). In general, asingle substrate will contain a network of adjacent target portions thatare successively exposed. Known lithographic apparatus include steppers,in which each target portion is irradiated by exposing an entire patternonto the target portion at once, and scanners, in which each targetportion is irradiated by scanning the pattern through the beam ofradiation in a given direction (the “scanning” direction) whilesynchronously scanning the substrate parallel or anti-parallel to thisdirection.

In a lithographic apparatus the size of features that can be imaged ontothe substrate is limited by the wavelength of the projection radiation.To produce integrated circuits with a higher density of devices, andhence higher operating speeds, it is desirable to be able to imagesmaller features. While most current lithographic projection apparatusemploy ultraviolet light generated by mercury lamps or excimer lasers,it has been proposed to use shorter wavelength radiation, e.g. of around13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray,and possible sources include, for example, laser-produced plasmasources, discharge plasma sources, or synchrotron radiation fromelectron storage rings.

Some EUV sources, especially plasma sources, emit radiation over a widerange of frequencies, even including infrared (IR), visible, ultraviolet(UV) and deep ultraviolet (DUV). These unwanted frequencies willpropagate and cause heating problems in the illumination and projectionsystems and cause unwanted exposure of the resist if not blocked;although the multilayer mirrors of the illumination and projectionsystems are designed for reflection of the desired wavelength, e.g. 13nm, they are optically flat and have quite high reflectivities at IR,visible and UV wavelengths. It is therefore necessary to select from thesource a relatively narrow band of frequencies for the beam ofradiation. Even where the source has a relatively narrow emission line,it is desirable to reject radiation out of that line, especially atlonger wavelengths. It has been proposed to use a thin membrane as afilter to perform this function. However, such a film is very delicateand becomes very hot, 200-300° C. or more, leading to high thermalstresses and cracking, sublimation and oxidation in the high powerlevels necessary in a lithographic projection apparatus. A membranefilter also generally absorbs at least 50% of the desired radiation.

U.S. Pat. No. 6,678,037 describes a lithographic projection apparatuswherein a grating spectral filter is used in the radiation system of thelithographic projection apparatus. This grating spectral filter isdesigned for passing radiation of desired wavelengths to form a beam ofradiation and for deflecting radiation of undesired wavelengths. Thegrating spectral filter is substantially formed of a material having acomplex refractive index close to unity at the desired wavelengths andincludes silicon protrusions (this structure is ‘invisible’ for the EUVradiation). The protrusions have a laminar sawtooth profile or a laminarsquare wave profile (FIGS. 3 and 4 of U.S. Pat. No. 6,678,037,respectively). Further, the structures might have a Ru coating creatingan rms surface roughness of 1 nm.

A problem with optical filters with such coatings is that they alsoreflect a large amount of desired radiation, whereas transmission(through the protrusions) is required.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide improved opticalelements, like filters, lenses, etc., having a layer that promotetransmission through at least a part of the optical element or decreasereflection of desired wavelengths. Such optical elements may be used ina lithographic projection apparatus to select EUV radiation from a wideband source and/or to reject unwanted frequencies, but may also be usedin other applications, e.g. in the UV or visible light (VIS).

It is another aspect of the present invention to provide an opticalelement, for example an optical filter, an optical grating, a mirror, alens, etc., with a layer that has a decreased reflection of incoming EUVradiation with a wavelength λ in the range of 5-20 nm.

According to an embodiment of the present invention an optical elementincludes a layer that is at least partially transmissive for EUVradiation with a wavelength λ in the range of 5-20 nm, and a top layerincluding a structure having an rms roughness value, wherein the toplayer is transmissive for EUV radiation with wavelength λ in the rangeof 5-20 nm, and the structure of the top layer is a structure having anrms roughness value equal to or larger than λ/10 for spatial periodsequal to or smaller than λ/2.

Such an optical element reduces reflection of radiation (e.g. projectedwith a certain angle of incidence) on the surface of the opticalelement, especially on the transmissive top layer, due to the presenceof this transmissive top layer with a structure. Hence, more radiationis transmitted through the transmissive top layer and/or the (partially)transmissive layer of the optical element (see e.g. E. Spiller, Soft RayOptics, SPIE Optical Engineering Press, US, 1994, ISBN 0-8194-1655-x).In this way, more radiation reaches the optical element, and thus, theoptical element may provide a desired filter function.

In a further embodiment, the structure of the transmissive top layer hasan rms roughness equal to or smaller than λ for spatial periods largerthan λ/2 and equal to or smaller than 1 μm. In yet a further embodiment,the at least partially transmissive layer includes a surface with astructure with an rms roughness value equal to or larger than λ/50 andequal to or smaller than λ for spatial periods larger than about λ/2 andequal to or smaller than about 1 μm, or a structure with an rmsroughness value equal to or larger than about λ/50 and equal to orsmaller than about λ/2 for spatial periods larger than about λ/2 andequal to or smaller than about 1 μm.

In yet another embodiment, the transmissive top layer is composed of amaterial with a relatively low imaginary part of the index of refractionfor radiation with wavelength λ in the range of 5-20 nm. For example,for EUV applications, the transmissive top layer includes a materialselected from Be, B, C, Si, P, S, K, Ca, Sc, Br, Rb, Sr, Y, Zr, Nb, Mo,Ru, Rh, Ag, Ba, La, Ce, Pr, Ir, Au, Pa or U. All these elements have anattenuation length for EUV radiation larger than about 100 nm.Combinations of materials may also be chosen, for example B₄C or SiC,etc. In another embodiment, the transmissive top layer includes one ormore materials selected from Be, B, C, Si, Zr, Nb, Mo or Ru. Thematerial of the structure may be different from the material of thetransmissive top layer. However, they may also be the same.

In still another embodiment, the transmissive top layer includes atleast Ru. Ru is relatively more stable to oxidation than, for example,Si. In this way, subwavelength roughness is introduced by a relativelystable transmissive top layer including the structure. Such transmissivetop layer also provides protection of the optical element, for exampleto oxidation. In another embodiment, the transmissive top layer and thestructure include Ru. In U.S. Pat. No. 6,678,037, the Ru layer may onlyprovide protection but the presence of this Ru layer in U.S. Pat. No.6,678,037 also enhances reflection. In contrast to U.S. Pat. No.6,678,037, the transmissive top layer including the structure accordingto the invention may both protect and diminish reflection that mayotherwise be generated by such protective layer, as in U.S. Pat. No.6,678,037. Other materials, next to Ru, that may be chosen for thetransmissive top layer of the invention and which may provide arelatively chemically inert layer are Au, Rh, Ir, Ag, C, etc.

In a further embodiment, the structure having an rms roughness valueequal to or larger than λ/10 has spatial periods equal to or smallerthan λ/2, for example λ/5 or smaller. When the spatial period of thestructure is below λ/2, then the reflection loss is ‘converted’ intotransmission gain. For EUV radiation of about 13.5 nm, the spatialperiod may be about 6 nm or smaller, 5 nm or smaller, 2.5 nm or smaller,2 nm or smaller, or equal to or smaller than about 1 nm. In anotherembodiment, the transmissive top layer may be a layer wherein the rmsvalue is about λ/5 or larger, or λ/2 or larger, or λ or larger, or 2*λor larger, e.g. about 2 nm or larger, 2.5 nm or larger, 5 nm or larger,6 nm or larger, 10 nm or larger, or about 13.5 nm or larger for EUVradiation of about 13.5 nm or even larger, e.g. 20 or 50 nm for abovementioned spatial period equal to or smaller than λ/2. Good results forEUV applications may be obtained with rms roughness values between 1.5and about 50 nm, e.g. 2-20 nm. Further, this rms roughness between about1.5 and about 50 nm may be selected for spatial periods equal to orsmaller than λ/2, e.g. between about 10-2.5 nm (upper values (λ/2)) andabout 1.0-4 nm (lower values (λ/5)).

In yet a further embodiment, the structure of the top layer is astructure having an rms roughness value equal to or larger than aboutλ/10 and equal to or larger than about 4*λ, for spatial periods equal toor smaller than about λ/2. In a still further embodiment, the structureof the top layer is a structure having an rms roughness value equal toor larger than about λ/10 and equal to or larger than about 4*λ, forspatial periods equal to or smaller than about λ/2 and equal to orlarger than about λ/5.

In yet a further embodiment, there is provided an optical elementincluding the top layer with a structure, the structure having an rmsroughness value equal to or larger than about λ/10 and equal to orsmaller than about 4*λ, for spatial periods equal to or smaller thanabout λ/2 and equal to or larger than about λ/5; and an rms roughnessvalue equal to or larger than about λ/50 and equal to or smaller thanabout λ for spatial periods larger than about λ/2 and equal to orsmaller than about 1 μm.

Spatial periods and rms roughness can be determined by existingtechniques, for example optical scattering techniques, STM, AFM,interferometrics, etc.

In yet another embodiment, the transmissive top layer includes a layerwith a thickness of up to about 10 nm, or up to about 20 nm, for exampleabout 10-20 nm.

In another embodiment, the at least partially transmissive layer, i.e.the layer over which the transmissive top layer of the invention ispresent, has the surface with the structure with a certain roughness.This transmissive layer may be a lens, a layer on a mirror, for examplea grazing incidence mirror including such at least partiallytransmissive layer, for example a Mo layer, or a foil, etc. Suchtransmissive or reflective layers are usually polished or otherwisesurfaced, such that a certain maximum roughness is obtained. Forexample, an optical element with a partially transmissive layer with asubstantially flat surface may be applied, wherein the substantiallyflat surfaces of the partially transmissive layer includes a surfacewith a structure with an rms roughness value equal to or smaller than λ.The rms roughness may also be equal to or smaller than about λ/2 orequal to or smaller than about λ/5. On top of this substantially flatsurface, the transmissive top layer including the structure according tothe present invention is present.

In a further embodiment, the optical element of the invention may alsoinclude a layer with transmissive protrusions, for example in the caseof a reflective mirror, having a mirroring surface, including one ormore protrusions transmissive for EUV radiation with wavelength λ in therange of 5-20 nm. The transmissive top layer according to the presentinvention with the structure may be present on those protrusions, on thelayer or surface between the protrusions or on both protrusion and layeror surface between the protrusions. Due to the presence of thistransmissive top layer, less radiation with wavelength λ in the range of5-20 nm may be reflected and more radiation may be guided to the opticalelement, for example through the transmissive protrusions.

In a further embodiment, the optical element includes a mirror, having amirror surface, wherein the mirror surface includes one or moreprotrusions transmissive for EUV radiation with wavelength λ in therange of 5-20 nm, and at least part of the mirror surface furtherincludes the transmissive top layer including the structure. Such anoptical element may be according to European Patent Application03077155, incorporated herein by reference. In a variation of thisembodiment, the mirror surface includes protrusions including a materialselected from Be, B, C, Si, P, S, K, Ca, Sc, Br, Rb, Sr, Y, Zr, Nb, Mo,Ru, Rh, Ag, Ba, La, Ce, Pr, Pa or U and combinations thereof.

In a further variation on this embodiment, the mirror surface includesone or more first protrusions including a first material selected fromBe, B, C, P, S, K, Ca, Sc, Br, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Ag, Ba,La, Ce, Pr, Pa or U and combinations thereof, and one or more secondprotrusions including a second material selected from Be, B, C, Si, P,S, K, Ca, Sc, Br, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Ag, Ba, La, Ce, Pr, Paor U and combinations thereof, and the first and second materials arenot the same. On top of these first and/or second protrusions, thetransmissive top layer according to the present invention is provided.

In another embodiment, an optical element with at least one surfaceincludes a profile having height differences, thereby providing cavitiesand elevations having a predetermined maximum height difference, whereinthe optical element further includes a substantially flat transmissivelayer in the cavities and on the elevations of the optical element,according to U.S. patent application Ser. No. 10/936,716, filed Sep. 9,2004, incorporated herein by reference, and the optical element furtherincludes the transmissive top layer including the structure according tothe present invention, for example on the substantially flattransmissive layer.

According to another embodiment, a lithographic apparatus includes oneor more optical elements according to the present invention. Suchlithographic apparatus may include an illumination system configured toa beam of radiation; a support configured to support a patterningdevice, the patterning device configured to pattern the beam ofradiation according to a desired pattern; a substrate table configuredto hold a substrate; and a projection system configured to project thepatterned beam onto a target portion of the substrate.

In a further aspect, the present invention is directed to a devicemanufacturing method including providing a beam of radiation; patterningthe beam of radiation with a pattern in its cross-section; projectingthe beam of radiation onto a target portion of a substrate after itspatterned, wherein the beam of radiation is incident on an opticalelement according to the present invention.

According to yet another aspect of the invention there is provided adevice which is manufactured according to the method of the presentinvention.

In the context of the present invention, “optical element” includes oneor more of optical filters, optical gratings, mirrors and lenses. Suchoptical elements may be flat or curved and may be present as layer,foil, device, etc. They may be blazed or optimised, e.g. for radiationwith a predetermined wavelength λ in the range of 5-20 nm. They may betransmissive for radiation with wavelengths λ, e.g. in the case oflenses, or reflective, e.g. in case of mirrors, or diffractive, e.g. inthe case of gratings. Some optical elements may provide one or more ofthese optical effects; see e.g. U.S. patent application Ser. Nos.10/887,306 and 10/936,716. The optical elements of the present inventionat least include a partially transmissive layer and a transmissive toplayer including a structure. It should be appreciated that the opticalelement may also include other layers, features, etc. These layers mayalso be between the partially transmissive layer and a transmissive toplayer.

“Transmissive” or “substantially transmissive” in this context meansthat the transmission through a transmissive layer, e.g. a EUVtransmissive layer, is larger than zero, for examplet least 30%, or atleast 50%, at least 70%, at least 80%, at least 90%, or at least 95%, orat least 98%. Alternatively, transmissive may also mean that attenuationthrough the transmissive layer or through a transmissive protrusion issmaller than about 1/e (0.368). Herein, attenuation length is defined asthe length wherein the transmittance has reduced to a value of 1/e, aswould be understood by one of ordinary skill in the art.

“Not absorbed” or “substantially not absorbed” in this context meansthat the absorption of radiation is less than 100%, or less than 70%, orless than 50%, or less than 30%, or less than 20%, or less than 10%, orless than 5%, or less than 2%.

It should be appreciated that “transmissive” as well as “not absorbed”does not only depend on the transmission or absorbance of the material,but also on other factors, for example layer thickness. For example, dueto the fact that the transmissive layer including a structure accordingto the present invention may be relatively thin, also relatively lesstransmissive materials like Ru, Au etc. may be applied here,transmissive with respect to EUV radiation.

“Undesired radiation” or “undesired wavelength” refers to radiationhaving wavelengths larger or smaller than the wavelength that isintended to be used. For example, when EUV radiation with a wavelengthof about 13.5 nm is desired, radiation with a wavelength smaller thanabout 10 nm or larger than about 20 nm is not desired. This means thatthe phrase “radiation with wavelength λ in the range of 5-20 nm” is notintended to be limited to radiation with an infinite small bandwidth ofλ, as will be understood by those of ordinary skill in the art. Anoptical element may be designed for one specific wavelength λ or a rangeof wavelengths. An optical element may also be used at differentwavelengths, e.g. due to second order effects, etc.

In the context of the invention, a “structure” is defined as a structurewhich may be present on a layer or on a surface, or which can be seen assurface, providing height differences. The structure may includerandomly ordered height differences (random structure) or ordered heightdifferences (e.g. one or two dimensional ordering of the heightdifferences).

In the context of the invention, a “random structure” is defined as astructure which may be present on a layer or on a surface, or which canbe seen as surface, and which does not include a regular pattern or doesnot include an intended regular pattern or profile. For example, aftersurfacing, for example polishing, there might be found some regularity,but this regularity may not be intentional. In contrast to that, in thecase of gratings, the protrusions forming the grating profile have anintended regular pattern. A random structure may be obtained bydeposition sputtering or otherwise depositing structures of a certaindimension (e.g. 2, 5 or 10 nm diameter) on a surface. This will appearin a stochastic way. The layer including a random structure may also beobtained by depositing a layer and subsequently sputtering part of thelayer away.

The term “layer” describes layers having one or more boundary surfaceswith other layers and/or with other media like vacuum (in use). However,“layer” may also mean part of a structure, as will be understood bythose of ordinary skill in the art. The term “layer” may also indicate anumber of layers. These layers can be next to each other or on top ofeach other, etc. They may include one material or a combination ofmaterials. “Layers” may be continuous or discontinuous layers. Forexample, protrusions on a surface may also be seen as separate layers oras a discontinuous layer. This means that the layer that is at leastpartially transmissive for EUV radiation according to the invention mayinclude protrusions.

The “transmissive top layer” of the present invention includes thestructure, but may also essentially only include the structure. Thetransmissive top layer of the present invention may be a separate layer,for example on protrusions or on a transmissive layer, but may also bepart of the at least partially transmissive layer of the opticalelement. At least part of the transmissive top layer of the inventionincludes the structure according to the present invention.

In the present invention, the “at least partially transmissive layer” ofthe optical element is a layer that is (substantially) transmissive fordesired radiation with wavelength λ in the range of 5-20 nm. “Partly” inthis context describes the fact that the layer is not necessarilycompletely transmissive, for example at the boundaries. Thetransmissiveness of this layer will depend on, for example, thewavelength of the radiation and angle of incidence of the beam ofradiation, as will be understood by those of ordinary skill in the art.This also means that a layer can both be reflective and transmissive.Next to that, depending on the wavelength, angle of incidence of thebeam of radiation and a possible ordering (like e.g. a grating) of thislayer, the layer may also have refractive effects. However, whenapplying an optical element according to the present invention, one ofthese optical effects will mainly be used with respect to radiation withwavelength λ in the range of 5-20 nm, hereby not excluding the possibleuse of other effects and/or at other wavelengths than at λ. The “atleast partially transmissive layer” in the present invention may includeordered protrusions, a layer on and/or between protrusions, a lens, orlayer on a lens, etc.

In the invention, “protrusions”, which are present on the mirrorsurface, are defined as structures extending from the mirror surfacethat may include a material selected from Be, B, C, Si, P, S, K, Ca, Sc,Br, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Ag, Ba, La, Ce, Pr, Pa or U andcombinations thereof These protrusions may be made via lithographictechniques. In such a technique, the upper surface is polished to a verygood surface roughness and grooves are etched with the lands defined bya photoresist pattern. In this way, a certain profile is obtained, forexample a laminar block or laminar sawtooth. The profile may also beprovided by ruling (scratching a line in) a layer of a first materialwith a diamond tool.

These protrusions on a mirror surface may have a laminar sawtoothprofile, wherein the protrusions have a sawtooth profile that islaminarly arranged on the mirror surface. The profile can be arranged toform a blazed one dimensional grating, with a certain number of parallellines (laminar sawtooth protrusions). The protrusions may also have alaminar square wave profile, wherein the protrusions have a square orrectangle structure, that is laminarly arranged on a mirror surface. Theprofile can be arranged to form a one dimensional grating, with acertain number of parallel lines (laminar square wave protrusions).

The protrusions might also be periodically arranged in two directions.For example, the protrusions may have periodically structured sawtoothprofile, wherein the protrusions may be cubes or rectangles which have asawtooth profile in one direction, and which are periodically arranged,like a checkerboard. The profile can be arranged to form a blazed twodimensional grating, with a certain number of periodically arrangedstructures (periodical sawtooth protrusions). A further embodiment of aperiodically arranged profile in two directions is a structure with aperiodically structured square wave profile, wherein the protrusions cane.g. be cubes or rectangles which are periodically arranged, like acheckerboard. The profile can be arranged to form a blazed twodimensional grating, with a certain number of periodically arrangedcubes or rectangles (periodical square wave protrusions). When usingsuch two dimensional profiles, the protrusions may be arranged in ablock structure of sawtooth protrusions (free standing periodicalsawtooth protrusions) or block protrusions (free standing periodicalsquare wave protrusions; with cubes or rectangles), as described forexample in U.S. Pat. No. 6,469,827 or E. Hecht, “Optics”, secondedition, p. 430 (paragraph 10.2.7).

In the present invention, calling a protrusion “first” and “second”protrusion is only a method to indicate the different protrusions ofdifferent materials. These terms do not imply a certain order.

The protrusions or elevations form a “profile”, for example a grating,and provide on the surface of such optical elements “cavities” (areaswhich are deeper with respect to adjacent areas), and which can be seenas the area (two dimensionally speaking) between protrusions orelevations (areas which are higher with respect to adjacent areas). Theprotrusions may be flat and have equal heights. The cavities may be flatand have equal depths and may thus be equal to the height of theprotrusions. This means that the height of the protrusions is apredetermined maximum height difference. In case the protrusions andcavities are not flat, the maximum height difference between the bottomsurface of the cavities and the top surface of the elevations is thepredetermined maximum height. In general, the maximum height differencebetween the top surface of the elevation and the bottom surface of thecavity may be determined and is the “predetermined maximum heightdifference”.

In the context of the present invention, “first height” means the heightin the cavities which is obtained after providing a transmissive layerin the cavities and on the elevations of the optical elements, butbefore surfacing. Due to a subsequent surfacing procedure, the firstheight is reduced such that a height in the cavities is obtained that islarger than the predetermined maximum height difference. Further, theterm “second height” refers to the height of the transmissive layer,e.g. a EUV transmissive layer, on the elevations that is obtained aftersurfacing, e.g. polishing.

“Substantially zero” means that the second height of the layer above theprotrusions or elevations may be reduced, within practical limits ofsurfacing methods, to substantially zero, e.g. a few nanometers or less.Before surfacing, the transmisive layer in the cavities has a firstheight. When surfacing, e.g. polishing or etching, the height of thetransmissive layer is reduced, and thus the first height is reduced too,such that the second height on the elevations that is obtained aftersurfacing is substantially zero or larger. When the height of thetransmissive layer is reduced until the height over the elevations(second height) is substantially zero, the first height is also reduced,resulting in a final height in the cavities that is “substantiallyequal” to the predetermined maximum height difference. When the layerheight in the cavity is substantially equal to the predetermined maximumheight difference, there may be a difference of e.g. only a fewnanometers or less between these heights.

The phrase “a substantial planar surface in the cavities and over theelevations” describes the situation that a substantially continuoustransmissive layer is present on at least part of the optical element,thereby providing such layer in the cavities and on the elevations.

In the present invention, “material” may also be interpreted ascombination of materials. The phrase “material selected from Be, B, C,Si, P, S, K, Ca, Sc, Br, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Ag, Ba, La, Ce,Pr, Ir, Au, Pa or U” herein also includes materials including one ormore of these elements, for example silicon nitride Si₃N₄, boronnitrides BN, calcium nitrides Ca₃N₂, etc.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beappreciated that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin-film magnetic heads, etc. It shouldbe appreciated that, in the context of such alternative applications,any use of the terms “wafer” or “die” herein may be considered assynonymous with the more general terms “substrate” or “target portion”,respectively. The substrate referred to herein may be processed, beforeor after exposure, in for example a track (a tool that typically appliesa layer of resist to a substrate and develops the exposed resist) or ametrology or inspection tool. Where applicable, the disclosure hereinmay be applied to such and other substrate processing tools. Further,the substrate may be processed more than once, for example in order tocreate a multilayer IC, so that the term substrate used herein may alsorefer to a substrate that already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength λ of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g. having a wavelength in the range of5-20 nm), as well as particle beams, such as ion beams or electronbeams. Herein, the phrase “in the range of 5-20 nm” refers to radiationhaving a wavelength between at least part of 5-20 nm. It should beappreciated that the present invention may also be used for radiationhaving a wavelength in the EUV or soft X-ray outside this range, e.g.about 1 nm or 25 nm.

The term “patterning device” used herein should be broadly interpretedas referring to a device that can be used to impart a beam of radiationwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the beam of radiation may not exactly correspond to thedesired pattern in the target portion of the substrate. Generally, thepattern imparted to the beam of radiation will correspond to aparticular functional layer in a device being created in the targetportion, such as an integrated circuit.

Patterning devices may be transmissive or reflective. Examples ofpatterning device include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned.

The support supports, e.g. bares the weight of, the patterning device.It holds the patterning device in a way depending on the orientation ofthe patterning device, the design of the lithographic apparatus, andother conditions, for example whether or not the patterning device isheld in a vacuum environment. The support may use mechanical clamping,vacuum, or other clamping techniques, for example electrostatic clampingunder vacuum conditions. The support may be a frame or a table, forexample, which may be fixed or movable as required and which may ensurethat the patterning device is at a desired position, for example withrespect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device”.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “lens” herein may be considered assynonymous with the more general term “projection system”.

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the beam of radiationof radiation, and such components may also be referred to below,collectively or singularly, as a “lens”.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located, for example, between theprojection system and the substrate during exposure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings inwhich corresponding reference symbols indicate corresponding parts, andin which:

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the present invention;

FIG. 2 schematically depicts a side view of an EUV illumination systemand projection optics of a lithographic projection apparatus accordingto FIG. 1;

FIG. 3 schematically depicts a cross section of a theoretical roughsurface;

FIG. 4 a schematically depicts an optical filter having a mirror surface(reflective layer) and protrusions in the form of a laminar square waveprofile and a transmissive top layer according to an embodiment of thepresent invention;

FIG. 4 b schematically depicts an optical filter having a mirror surface(reflective layer) and protrusions in the form of a laminar sawtoothprofile and a transmissive top layer according to an embodiment of thepresent invention;

FIG. 4 c schematically depictss an optical filter having a mirrorsurface (reflective layer) and protrusions in the form of a laminarasymmetrical two-sided sawtooth profile and a transmissive top layeraccording to an embodiment of the present invention;

FIG. 5 schematically depicts a mirror with mirror surface protrusions ina form of a laminar square wave profile, a substantially flat (EUV)transmissive layer and a transmissive top layer according to anembodiment of the present invention;

FIG. 6 schematically depicts a mirror with a tilted multilayer stack andan (EUV) transmissive layer on top of this stack and a transmissive toplayer according to an embodiment of the present invention;

FIG. 7 schematically depicts a mirror with mirror surface protrusions inthe form of a laminar sawtooth profile according to an embodiment of thepresent invention;

FIG. 8 schematically depicts a mirror with mirror surface protrusions inthe form of a laminar square wave profile according to an embodiment ofthe present invention;

FIG. 9 schematically depicts a similar mirror as shown in FIG. 4,wherein the protrusions are arranged in such a way that a part of EUVradiation of a beam of radiation including EUV radiation, only passesone protrusion, according to an embodiment of the present invention;

FIG. 10 schematically depicts a mirror with mirror surface protrusionsin the form of a laminar sawtooth profile of two materials on top ofeach other, according to an embodiment of the present invention; and

FIG. 11 schematically depicts a mirror with mirror surface protrusionsin the form of a laminar square wave profile of two materials next toeach other, according to an embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus 1 according to thepresent invention. The apparatus 1 includes a base plate BP. Anillumination system (illuminator) IL is configured to provide a beam ofradiation PB of radiation (e.g. UV or EUV radiation). A first support(e.g. a mask table) MT is configured to support a patterning device(e.g. a mask) MA and is connected to a first positioning device PM thataccurately positions the patterning device with respect to a projectionsystem PL. A substrate table (e.g. a wafer table) WT is configured tohold a substrate (e.g. a resist-coated wafer) W and is connected tosecond positioning device PW that accurately positions the substratewith respect to the projection system PL. The projection system (e.g. areflective projection lens) PL is configured to image a pattern impartedto the beam of radiation PB by patterning device MA onto a targetportion C (e.g. including one or more dies) of the substrate W.

As here depicted, the apparatus is of a reflective type (e.g. employinga reflective mask or a programmable mirror array of a type as referredto above). Alternatively, the apparatus may be of a transmissive type(e.g. employing a transmissive mask).

The illuminator IL receives radiation from a radiation source SO. Thesource and the lithographic apparatus may be separate entities, forexample when the source is a plasma discharge source. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation is generally passed from the source SO to the illuminatorIL with the aid of a radiation collector including, for example,suitable collecting mirrors and/or a spectral purity filter. In othercases the source may be integral part of the apparatus, for example whenthe source is a mercury lamp. The source SO and the illuminator IL, maybe referred to as a radiation system.

The illuminator IL may include an adjusting device(s) configured toadjust the angular intensity distribution of the beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. The illuminator providesa conditioned beam of radiation PB having a desired uniformity andintensity distribution in its cross-section.

The beam of radiation PB is incident on the mask MA, which is held onthe mask table MT. Being reflected by the mask MA, the beam of radiationPB passes through the projection system PL, which focuses the beam ontoa target portion C of the substrate W. With the aid of the secondpositioning device PW and a position sensor IF2 (e.g. an interferometricdevice), the substrate table WT can be moved accurately, e.g. so as toposition different target portions C in the path of the beam PB.Similarly, the first positioning device PM and a position sensor IF1(e.g., an interferometric device) can be used to accurately position themask MA with respect to the path of the beam PB, e.g. after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe object tables MT and WT will be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which form part of the positioning devices PM and PW.However, in the case of a stepper, as opposed to a scanner, the masktable MT may be connected to a short stroke actuator only, or may befixed. Mask MA and substrate W may be aligned using mask alignment marksM1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in the following preferred modes:

-   1. In step mode, the mask table MT and the substrate table WT are    kept essentially stationary, while an entire pattern imparted to the    beam of radiation is projected onto a target portion C at once (i.e.    a single static exposure). The substrate table WT is then shifted in    the X and/or Y direction so that a different target portion C can be    exposed. In step mode, the maximum size of the exposure field limits    the size of the target portion C imaged in a single static exposure.-   2. In scan mode, the mask table MT and the substrate table WT are    scanned synchronously while a pattern imparted to the beam of    radiation is projected onto a target portion C (i.e. a single    dynamic exposure). The velocity and direction of the substrate table    WT relative to the mask table MT is determined by the    (de-)magnification and image reversal characteristics of the    projection system PL. In scan mode, the maximum size of the exposure    field limits the width (in the non-scanning direction) of the target    portion in a single dynamic exposure, whereas the length of the    scanning motion determines the height (in the scanning direction) of    the target portion.-   3. In another mode, the mask table MT is kept essentially stationary    holding a programmable patterning device, and the substrate table WT    is moved or scanned while a pattern imparted to the beam of    radiation is projected onto a target portion C. In this mode,    generally a pulsed radiation source is employed and the programmable    patterning device is updated as required after each movement of the    substrate table WT or in between successive radiation pulses during    a scan. This mode of operation can be readily applied to maskless    lithography that utilizes programmable patterning device, such as a    programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 shows the apparatus 1 in more detail, including a radiationsystem 42, an illumination optics unit 44, and the projection system PL.The radiation system 42 includes the radiation source SO which may beformed by a discharge plasma. EUV radiation may be produced by a gas orvapor, such as Xe gas or Li vapor in which a very hot plasma is createdto emit radiation in the EUV range of the electromagnetic spectrum. Thevery hot plasma is created by causing a partially ionized plasma of anelectrical discharge to collapse onto an optical axis O. Partialpressures of e.g. 10 Pa of Xe or Li vapor or any other suitable gas orvapor may be required for efficient generation of the radiation. Theradiation emitted by radiation source SO is passed from a source chamber47 into a collector chamber 48 via a gas barrier or contamination trap49. The gas barrier 49 includes a channel structure, for exampleasdescribed in detail in U.S. Pat. Nos. 6,614,505 and 6,359,969, whichare incorporated herein by reference.

The collector chamber 48 includes a radiation collector 50 which may beformed by a grazing incidence collector. Radiation passed by collector50 is reflected off a grating spectral filter 51 to be focused in avirtual source point 52 at an aperture in the collector chamber 48. Fromcollector chamber 48, a beam of radiation 56 is reflected inillumination optics unit 44 via normal incidence reflectors 53, 54 ontoa reticle or mask positioned on reticle or mask table MT. A patternedbeam 57 is formed which is imaged in projection optics system PL viareflective elements 58, 59 onto wafer stage or substrate table WT. Moreelements than shown may generally be present in illumination optics unit44 and projection system PL.

Radiation collectors 50 are known. One example of a radiation collectorthat may be used in the present invention is described in U.S. PatentApplication Publication 2004/0094724 A1. See, for example, FIGS. 3, 4and 5.

Having generally described a lithographic apparatus, a few general notesare made on roughness of surfaces. A rough surface generally includes astructure that provides a height profile. The structure includes smallerstructures, thereby providing height differences. The height profile ofsuch a surface may be provided by known techniques. The height profilecan be defined as the difference of the local height with respect to theaverage height of the profile. Since structures on rough surfaces may berandomly positioned, it is not necessary to define the fully detailedheight profile, i.e. it is not necessary to know the height of theprofile at every position. A set of numbers describing the key featuresof the structure is adequate. A convenient set of numbers is the rootmean square (rms) roughness value of the height for a set ofcharacteristic lengths over the surface. This is illustrated in FIG. 3,wherein a schematic cross section of a rough surface profile(irregularly curved thick line) is depicted. In FIG. 3, five sinefunctions are plotted which are included in the rough surface profile.The structures in the profile appear as randomly ordered. It is clearthat the long wavelength sine is still dominant in the profile.

In FIG. 3, the relative amplitude of the sine functions is chosen to beproportional to the wavelength. Table 1 sums up these relative numbers:

TABLE 1 the wavelengths and amplitudes of the sine waves in FIG. 3:Spatial period (nm) Amplitude (a.u.) 100 1 50 0.5 25 0.25 12.5 0.1256.25 0.0625The sine wave with the longest wavelength describes the relatively slowvariation of the height. Furthermore, the shorter the wavelength of thesine wave, the faster the variation of the height it describes is. Thestructure of FIG. 3 may be repeated a number of periods, therebyproviding a kind of regular structure, with the desired rms roughnessaccording to the present invention.

The profile in FIG. 3 can thus be characterised by the five sets ofnumbers (Fourier analysis of the profile) given in Table 1. Note that inthis case, the numbers in Table 1 completely define the profile in FIG.3. In general, this is not the case, since more than, for example five,wavelengths are needed for the decomposition. The amplitude of the sinewave as a function of the wavelength is referred to as ‘power spectraldensity’ in literature. The wavelength is also called ‘spatial period’.Another term that is frequently used is ‘spatial frequency’, thisfrequency is equal to 1 over the spatial period. Fourier analysis of thestructure or height profile of a rough surface provides the powerspectral density of the height profile.

The amplitudes of the sine waves, along with their spatial periods areenough to characterise the height profile of the structure. Therefore,in order to characterise the height profile of the structure, theprofile is decomposed it into a set of sine waves with properly chosenspatial periods. The corresponding amplitudes then characterise thestructure. In literature, roughness is frequently characterised usingthe root mean square value of the sine wave, i.e. ½√2 times theamplitude of the wave.

The spatial period (i.e. the wavelength) of the roughness is a quantityto be considered for optical components. For EUV mirrors, roughness withspatial frequencies smaller than 1 mm⁻¹ (i.e. spatial periods largerthan 1 mm) may result in problems with the image quality as resolutionand distortion. Mid-spatial frequency roughness (1-10⁻³ mm⁻¹, i.e.spatial periods between 1 μm and 1 mm) results in flare, and highspatial frequency roughness (10⁻³-mm⁻¹, i.e. spatial periods larger than1 μm) appears to result in reflection loss. Hence, the physical effectof roughness depends heavily on the spatial frequency of the roughness.Note that for real surfaces, the roughness is characterised or specifiedover a range of spatial periods, since a single spatial period does notdescribe the whole effect of roughness. This also means that the rmsvalues are calculated using not a single sine wave, but a superpositionof sine waves with spatial periods in a certain bound range.

The present invention is based on the fact that a surface with very fineroughness, i.e. with spatial periods (wavelengths) smaller than abouthalf of the wavelength of the incident light, may not produce, orproduces less, scattered light, and the loss in reflectivity may bepartially compensated by an increase in the transmission.

For example, when used in EUV lithography, the wavelength of the lightmay be 13.5 nm. Thus the spatial period of the roughness may be lowerthan about 6.75 nm. In order to have approximately all the lighttransmitted through the structure, significant rms roughness isrequired. For example, at 15 degrees angle of incidence on a Rustructure, an 8 nm rms roughness results in approximately 36% reflectionloss, i.e. more than about 36% of the incident light is transmitted. Ahigher roughness may result in an even higher transmission. Roughnesswith spatial periods larger than about 6.75 nm will result in morescattering, and hence more loss of desired radiation.

One or more of the mirrors of FIG. 2, for example the filter 51, may bean optical element, as depicted in FIG. 4 a, including a minor oroptical filter 300, having a mirror surface or reflective layer RL,wherein the mirror surface includes one or more protrusions 301(substantially) transmissive for EUV radiation with a wavelength in therange of 5-20 nm. See also U.S. patent application Ser. No. 10/887,306,filed Jul. 9, 2004, incorporated herein by reference. The mirror surfacefurther includes a transmissive top layer TL including a structure RSaccording to the present invention.

Such an optical filter or mirror 300 may have a protrusion profile witha laminar square wave profile as shown in FIG. 4 a, a laminar sawtoothstructure as shown in FIG. 4 b, or a laminar asymmetrical two-sidedsawtooth profile as shown in FIG. 4 c, etc.

FIG. 4 a schematically shows an optical filter or mirror 300, having areflective layer or mirror surface RL, wherein th mirror surface isprovided with a laminar square wave profile. The square wave profile isformed by protrusions 301 (G, groove; L, land) which have a period orpitch p of, for example, between 500-5000 nm, a height h of, forexample, 12-20 nm. On the protrusions 301, on surface 340, which areregularly arranged, a transparent top layer TL is provided, with, onthis layer or in this layer or as part of the layer TL, a randomlyarranged structure RS. Distances hd between the structures included instructure RS and the heights and diameters thereof might vary over thetransparent layer TL and over the protrusions 301 of optical filter ormirror 300 comparable to the structure shown in FIG. 3 thereby providingthe transmissive top layer TL with structure RS.

The structure RS may have an rms roughness value equal to or larger thanλ/10, for example about 1.5 nm, for spatial periods equal to or smallerthan λ/2, or about 6.75 nm or less for EUV radiation. However, thespatial periods might also be smaller, for example 3 or 1 nm, and therms value may be about 5 or 10 nm. Generally, a higher rms value forthese spatial periods leads to a better result, since the reflectionloss at the surface at constant wavelength and under the same angle ofincidence of the radiation decreases with an increasing rms roughness.

Referring to FIGS. 4 a and 4 b, a beam of radiation PB having an angleof incidence α with mirror 300 travels through the transmissive toplayer TL, including structure RS, reaches mirror surface RL and isreflected back with an angle β. Note that in the drawing no account istaken of possible diffraction or refraction. Angle α may be smaller orlarger than drawn in this figure. Angle α may be equal to angle β, butmay also be different, depending on the indices of refraction of thematerials of RS, TL, protrusions 301 and the medium over mirror 300.

Part of the radiation of beam of radiation PB may be reflected at thesurface of the protrusion, which is shown by reflected beam PBr. Due tothe presence of the transmissive top layer TL with structure RSaccording to the present invention, th reflected beam PBr is minimisedand more radiation of beam of radiation PB having the desired wavelengthλ in the range of 5-20 nm reaches reflective layer RL. If the top layerTL having structure RS according to the present invention wer notpresent, more radiation having desired wavelengths λ would have beenreflected, leading to loss of energy. In this way, desired radiation maybe more effectively reflected, whereas radiation having undesiredwavelengths may be absorbed, refracted and/or diffracted by protrusions301. Hence, by providing this transmissive top layer TL having structureRS according to the present invention, an improved optical filter may beprovided. When structure RS according to the present invention is notpresent, more radiation may be reflected, for example by thetransmissive and/or protective layer TL without such structure accordingto the invention, as is the case in, for example U.S. Pat. No.6,678,037.

Referring to FIG. 4 a, the transparent layer or top coat TL includingthe structure RS is not depicted on the vertical sides of theprotrusions 301 or on the reflective surface RL in between protrusion301. Neither has this been shown in FIG. 4 b on the sides opposite ofthe angle ba. However, variations of the embodiment shown in FIGS. 4 a-balso include the situation in which the sides or surfaces include atransparent top layer TL according to the present invention, as shown,for example, in FIG. 4 c. The sides or surfaces do not necessarily alsoinclude structure RS.

The transmissive top layer may be provided by a CVD process. Aftercreating a smooth top layer TL of, for example, about 2.5 nm, the CVDprocess may be followed by a ‘rough’ deposition of larger particles, forexample about 5-10 nm, thereby creating structures RS in and on thetransmissive top layer. The materials that are used during these twosteps of the CVD process might be different, but may also be the same.The fact that sputtering by ions changes the roughness of a layer isknown. Furthermore, sputter deposition and evaporation depositiontechniques result in a certain roughness of a sample, depending on thedeposition settings. Polishing of a layer can also be used, depending onthe required roughness, the grain of the polishing material (fluid withparticles) can be chosen. With optical methods, the obtained roughnesscan be evaluated. After this deposition, a second CVD process may beapplied, to form a thin layer (not depicted in FIGS. 4 a-c) on thetransmissive top layer TL with structure RS. This layer may include thesame material as the transmissive layer TL and/or structure RS. Forexample, in a variation transmissive layer TL and structure RS includeRu, and in another variation, transmissive layer TL provided by firstand second CVD processes include Si and structure RS include Si.

In this embodiment and its variations, the at least partiallytransmissive layer TL, having a surface with the structure with an rmsroughness value equal to or smaller than λ, for example after polishing,are the protrusions 301 and their surfaces 340. On the sides of theprotrusions, extending from the at least partially transmissive layer,in FIGS. 4 a-c reflective layer RL, and when applicable on the surfaceof reflective layer RL between the protrusions, the groove G, thetransparent top TL layer may also be present.

Structure RS is depicted in the FIGS. 4 a-c (and 5 and 6) as sphericaland of equal, sizes. However, the structure may have different particlesizes, shapes, etc., for example a size distribution ranging from about0 to 20 nm, for example about 1-10 nm, or for example 2 or 5 nm. Thetransmissive top layer may have a thickness up to about 20, for exampleabout 1-10.

Some variations of mirror 300 with protrusions 301 are described below,which are independent of the transmissive top layer TL including astructure RS according to the present invention.

According to another embodiment, an optical element with at least onesurface includes a profile having height differences, thereby providingcavities and elevations having a predetermined maximum heightdifference, wherein the optical element includes a substantially flattransmissive layer in the cavities and on the elevations of the opticalelement according to U.S. patent application Ser. No. 10/936,716, andthe optical element further includes the transmissive top layerincluding the structure according to the present invention.

First, this embodiment is described in general independent of thetransmissive top layer TL according to the present invention, andsubsequently, the top layer TL and its function are discussed.

The mirror with the grating structure in this embodiment is described asEUV optical element, but this embodiment is not limited to EUVapplications.

By way of example, this embodiment is schematically depicted in FIG. 5,wherein a mirror 300 is shown having protrusions 301, wherein theprotrusions form a laminar square wave profile. The protrusions have aperiod p, a length 380, and a height 502. Height 502 is thepredetermined maximum height since all protrusions 301 have the sameheight. Reference L refers to “land” and reference G refers to “groove”.The protrusions 301 include, for example, EUV absorbers, for example, Cror TaN, or a Si/Mo multilayer. However, transmissive materials, forexample Zr, creating a phase difference, may be used. The protrusions301 have a height 502 and the cavities or grooves G are, according tothe present invention, filled with an EUV transmissive layer 504 withheight 506. The height of the EUV transmissive layer above theelevations or protrusions 301 is indicated with reference 507 (secondheight).

EUV transmissive layer 504, as shown in FIG. 5, may be provided bychemical vapor deposition of an EUV transmissive material, for exampleSi. After providing this layer in the cavities and on the elevations,the EUV transmissive layer is surfaced, for example polished, such thatan EUV transmissive layer is obtained with height 507 above theelevations or protrusions 301, or a height 506 above the cavities orgrooves G.

In this embodiment, the at least partially transmissive layer having asurface with a structure is the EUV transmissive layer 504, for example.after surfacing. The roughness of this layer, for example aftersurfacing, may have an rms roughness value equal to or smaller than λ,for example. λ/2 or equal to or smaller than λ/5.

The mirror according to FIG. 5 may be used as grating, wherein thegrating is used to select the desired wavelength and the EUVtransmissive layer provides a protective layer which may easily becleaned, and also provides an optical filter function by transmittingEUV radiation and absorbing part of the non-EUV radiation. The EUVtransmissive layer 504 may include a material selected from Be, B, C,Si, P, S, K, Ca, Sc, Br, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Ag, Ba, La, Ce,Pr, Pa or U. The EUV transmissive layer 504 may also include acombination of two or more of these materials. It should be appreciatedthat the appropriate materials for the EUV transmissive layer 504 andthe protrusions 301 may be chosen.

In an alternative arrangement, the EUV transmissive layer 504 includes amaterial with a complex index of refraction close to unity, for exampleSi. By choosing such a material for the EUV transmissive layer 504, aredesign of the grating is not necessary, as the EUV transmissive layer504 is invisible for the EUV radiation. The EUV radiation of the beam ofradiation PB may travel substantially undisturbed through EUVtransmissive layer 504, as this layer is substantially transmissive forEUV radiation, and in case of Si also has a complex index of refractionclose to unity. This is shown by way of example in FIG. 5, where ray r1of beam of radiation PB travels through the EUV transmissive layer andis reflected at the mirror 300 into ray r2, without being refracted. Incase materials or combinations of materials are chosen that have acomplex index of refraction not close to unity, rays r1 and r2 may berefracted, which may mean that a redesigned grating structure has to beused.

FIG. 5 depicts a mirror with a grating structure having a laminar squarewave profile. However, it should be appreciated that this embodiment mayalternatively relate to a mirror with a sawtooth profile having an EUVtransmissive layer, as well as two dimensional gratings having such anEUV transmissive layer.

The grating shown in FIG. 5, or other gratings which are included inthis embodiment, may be used as the reflective elements 53, 54, 57and/or 58 of the lithographic projection apparatus of FIG. 2.

Having described the mirror with a grating structure having a flattransmissive layer in general, the transmissive top layer TL includingstructure RS on top of the optical element is described.

On top of the flat transmissive layer 504, FIG. 5 shows the transmisivetop layer TL according to the present invention. The transmissive toplayer TL includes structure RS. A beam of radiation PB, with ray r1,travels through the transmissive top layer TL including structure RS andis reflected at the mirror 300 into ray r2. If the transmissive toplayer TL with structure RS were not present, ray r1 of beam of radiationPB may be partially reflected at the surface 340 (interface of theatmosphere, usually vacuum in case of EUV, above the optical element andthe surface of the flat transmissive layer 504), instead of entering thetransmissive layer 504. However, due to the presence of the transmissivetop layer TL with structure RS according to the present invention,reflection is reduced and the optical element of FIG. 5 can betterprovide its filter function of optically filtering undesired wavelengthsin favor of desired wavelength λ, for example in the range of 5-20 nm.IF the structures RS according to the present invention were notpresent, more radiation might have been reflected, for example by thetransmissive and/or protective layer TL without such structure RS. Thisembodiment provides the possibility of creating structure RS bypolishing, as described above.

According to another embodiment, a blazed grating as a mirror with amirror surface and a tilted multilayer stack which is tilted withrespect to the mirroring surface, for example with a plurality of tiltedmultilayer stacks which are tilted with respect to the mirroringsurface, and an EUV transmissive layer on top of this stack, accordingto U.S. patent application Ser. No. 10/936,716, is provided. Such amirror provides at least one surface including a profile having heightdifferences, thereby providing cavities and elevations having apredetermined maximum height difference, wherein the optical elementincludes a substantially planar transmissive layer in the cavities andon the elevations of the optical element. First, this embodiment isdescribed in general independent of the transmissive top layer accordingto the present invention, and subsequently, the top layer and itsfunction are discussed.

The blazed grating as a mirror with a tilted multilayer stack and an EUVtransmissive layer on top of this stack in this embodiment is describedas EUV optical element, but this embodiment is not limited to EUVapplications.

This embodiment describes a blazed grating as a mirror with a tiltedmultilayer stack and an EUV transmissive layer on top of this stack, asshown in FIG. 6. Tilted multilayer mirrors without the EUV transmissivelayer, and their production are known. See, for example, Seely et al.Applied Optics 40, vol. 31, page 5565 (2001).

In FIG. 6, the multilayer surface is tilted, and a kind of tilted stackstructure of multilayers is created, with protrusions 301 ofmultilayers, for example Si and Mo multilayers, with period p, and blazeangle ba. The tilted multilayer stack of protrusions 301 may be presenton, for example, a substrate or holder 310. The protrusions orelevations 301 of multilayers form elevations with varying heights.Hence, the maximum height of a protrusions is indicated by elevation top311. Further, cavities or grooves G are present. Here, the heightdifference between the bottom of the cavity G and the top 311 ofprotrusions 301 is height 502, which is the predetermined maximum heightdifference.

Over the cavities G and protrusions or elevations 301, an EUVtransmissive layer 504 is provided. The height of this EUV transmissivelayer 504 is indicated by reference 507 calculated from elevation top311, or height 506 calculated from the bottom of cavities or grooves G.

When desired, a layer 312 may be present, for example to provide anextra protection or an additional optical filter function. This extralayer 312 is not only applicable in this embodiment, but may also beapplied in the previous embodiments. Such layer may include Ru.

The EUV transmissive layer 504, for example of Si, may be provided bychemical vapor deposition, thereby providing an EUV transmissive layer504 with a height larger than 502, followed by a surfacing (e.g.polishing) procedure. In this way a multilayer mirror is provided whichcan reflect EUV radiation, and which may deflect, reflect and/or absorbnon-EUV radiation. For example, ray r1, which is radiation having an EUVwavelength (e.g. 13.5 mm, desired radiation) of beam of radiation PB,propagates undisturbed or substantially undisturbed through layer 312and EUV transmissive layer 504 and is reflected on protrusion 301 of thetilted multilayer stack into ray r2. Taking into account diffraction ofthe radiation from the beam of radiation PB by the grating (not depictedin FIG. 6), the angle of incidence with which the beam of radiationreaches the surface of this tilted multilayer and/or the angle ba may bechosen, thereby determining the direction in which ray r2 is reflected.

Radiation having other wavelengths than EUV wavelengths, e.g. VIS, orIR, is reflected at the surface of layer 312 (this reflection is notshown), or reflected at the surface of the EUV transmissive layer 504.This is shown in FIG. 6 with reference r3, describing a ray with anon-EUV wavelength which is reflected at the surface of the EUVtransmissive layer 504 as ray r4.

Part or all of the radiation may also be transmitted through layer 312,or through EUV transmissive layer 504, or through both layers. However,due to the differences of index of refraction between the materials(including vacuum) on both sides of the interfaces, ray r5, being a rayof beam of radiation PB having a non-EUV wavelength, is refracted. Dueto this refraction, rays r5 and r6 (ray r5 reflected at a multilayerstack protrusion 301) are refracted as indicated in FIG. 6.

Since the EUV transmissive layer 504 is transmissive for radiationhaving EUV wavelengths, but is substantially not transmissive forradiation having non-EUV wavelengths, part of the radiation havingnon-EUV wavelength may be absorbed. As a result of the reflection,refraction and absorption, radiation having non-EUV wavelengths isdiminished in the direction in which radiation having EUV wavelength(r2) is reflected (diffraction is not taken into account in theschematic drawing of FIG. 6). In this way, an optical filter is obtainedwith a substantially planar surface. The optical filter is easilycleaned and provides an optical filter for EUV radiation (e.g. 13.5 mm).

It should be appreciated that generally in the case where there is arange of predetermined maximum height differences 502, then thepredetermined maximum height difference would be the height difference502 between the deepest cavity and highest elevation or protrusion 301.Further, it should be appreciated that in such case the height 507 ofthe EUV transmissive layer on the elevations larger than zero (aftersurfacing) will generally be defined with respect to the top 311 of thehighest elevation or protrusion 301, thereby providing a planar EUVtransmissive layer over the protrusions and cavities.

It should also be appreciated that the present invention may also beapplied to curved optical elements, the optical elements having aprofile having height differences on curved surfaces, like curvedmirrors or curved mirrors with grating structures, thereby taking intoaccount that the transmissive layer will generally have the samecurvature.

Having described the mirror with a tilted multilayer stack and an EUVtransmissive layer on top of this stack in general, the transmissive toplayer TL including structure RS on top of the optical element isdescribed.

Referring to FIG. 6, the transmissive top layer TL is provided on layer312. However, layer 312 may be absent, such that the transmissive toplayer TL is on the planar transmissive layer 504, as in FIG. 5, i.e. onsurface 340 of transmissive layer 504. In another variation of thisembodiment, layer 312 may also be used as transmissive top layer TL,after providing this layer with structure RS, for example by surfacingtechniques or CVD techniques, etc. Also this embodiment provides thepossibility of creating structure RS by polishing, as described above.

Over the flat transmissive layer 504, FIG. 6 shows the transmisive toplayer TL according to the invention. Here, the transmissive top layer TLincludes structure RS. A beam of radiation PB, with ray r1, travelsthrough the transmissive top layer TL, including structure RS and isreflected (and/or refracted; not taken into account in this figure) atmirror 300 into ray r2. If the transmissive top layer TL with structureRS were not present, ray r1 of beam of radiation PB may be reflected atthe interface of the atmosphere above the optical element and thesurface of the layer 312, instead of entering the transmissive layer504. However, due to the presence of the transmissive top layer TL withstructure RS according to the present invention, reflection is reducedand the optical element of FIG. 6 can provide its improved filterfunction of optically filtering undesired wavelengths in favor ofdesired wavelength λ. Further, due to the presence of the structure,less radiation may be reflected at transmissive layer TL.

According to another embodiment, the surface 340 of transmissive layer504 is treated by. a known surfacing technique, for example polishing,etching, etc. In this way, structure RS is formed and transmissive layer504 includes transmissive top layer TL and structure RS according to theinvention.

According to another embodiment, a combination of materials is chosen:transmissive layer 504 substantially includes Si and transmissive toplayer TL with structure RS substantially includes Ru.

Table 2 describes the dependence of the reflection and transmission of atop layer according to the present invention with an rms roughness valueof 8 nm and spatial periods equal to or smaller than λ/2 as function ofthe angle of incidence for two different materials for EUV radiation of13.5 nm:

TABLE 2 Example of reflection and transmission of a top layer accordingto the invention Angle of incidence Material Reflection Transmission 10Ru 0.62 0.26 15 Ru 0.45 0.36 20 Ru 0.27 0.43 5 Au 0.5 0.33 10 Au 0.230.44 15 Au 0.09 0.43When the spatial period is larger than about λ/2, less transmission isobtained.

For example, assuming an rms roughness of about 8 nm and a radiationbeam with radiation having a wavelength of 13.5 nm, impinging on thesurface having the structure according to the invention with an rmsroughness of about 8 nm, reflection will be about 45%. In case the rmsroughness of 8 nm is found for a spatial period equal to or smaller thanλ/2, then transmission is about 36% and scattering (reflection in randomdirections) is negligible. However, when the rms roughness of 8 nm isfound for a spatial period larger than λ/2, but smaller than about 1 μm,the transmission is negligible and 36% is scattered. Hence, the presentinvention provides transmission gain due to the structure according tothe invention.

According to another embodiment, the transmissive top layer TL includingstructure RS according to the present invention is provided to themirrors which are disclosed in U.S. patent application Ser. No.10/887,306. On the surface, or on part of the surface of the mirrors,filters and/or gratings described as described herein, and includingtransmissive layer TL including structure RS according to the presentinvention, may be provided, thereby providing the optical filteringfunction of this layer to the mirrors, filters and/or grating.

Referring to FIG. 7, a laminar sawtooth profile is shown, and theprotrusions have a period p, a length 380, a height h, and an angle ba.FIG. 8 depicts another embodiment with a protrusion profile having alaminar square wave profile, wherein the protrusions have a period p, alength 380, and a height h. Reference L refers to “land” and reference Grefers to “groove”. The protrusions are arranged on a mirror surface ofmirror 300.

When EUV radiation is reflected by the mirror 300, for example under acertain angle, only the desired EUV radiation is reflected in thisangle, whereas undesired radiation, for example IR radiation, isabsorbed by above mentioned materials and/or is deflected or refractedin other directions. By way of example, this is illustrated in FIG. 8.The beam of radiation PB, having an angle of incidence α hits thesurface of a protrusion LP1 (laminar protrusion 1). Part of the beam maybe reflected (not shown) and part of the beam, or all of the beam mayenter the protrusion. Since the protrusion is transmissive for EUVradiation, this radiation propagates unabsorbed or substantiallyunabsorbed, whereas radiation with undesired wavelengths like UV or IRis substantially absorbed. The beam may further propagate to the nextprotrusion (second laminar protrusion) LP2. Reaching the left surface ofthis protrusion LP2, part of the radiation may again be reflected (notshown), and part of the radiation will propagate through protrusion LP2.Protrusion LP2 will also discriminate between EUV radiation andradiation with other wavelengths. When the beam of radiation PB reachesthe surface of the mirror 300 (e.g. a multilayer mirror) at position305, the beam of radiation PB is reflected with an angle β (β may be αin case of specular reflection on the surface of mirror 300). The beamof radiation PB may propagate further through protrusions LP2 and LP3.In this way, the beam of radiation PB will include a higher EUV/non-EUVradiation ratio after reflection by the mirror of the invention thanbefore the incidence on the mirror. What has been illustrated in FIG. 8with respect to the propagation of the beam of radiation PB may alsoapply for FIG. 7, where instead of laminar protrusions forming a laminarsquare wave profile like in FIG. 8, a laminar sawtooth profile is used.

Note that refraction of the beam of radiation PB is not depicted inFIGS. 7 and 8 (and later FIGS. 9 and 11). The above-mentioned ratio mayfurther be improved by using a blazed or optimised grating. The mirrorwith the above-mentioned protrusions may, even when it is optimised orblazed at, for example, UV wavelengths, block undesired wavelengths, forexample IR) but transmit EUV radiation, since Be, B, C, Si, P, S, K, Ca,Sc, Br, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Ag, Ba, La, Ce, Pr, Pa and U aresubstantially transmissive for EUV radiation, but substantially not forradiation having wavelengths other than EUV. Next to that, all thesematerials, including Si, have an absorption length longer than 100 nmfor 13.5 nm. The protrusions can also include combinations of materials,for example. B₄C or SiC.

The protrusions on mirror 300 of this embodiment are mainly present asan optical filter that is transmissive for EUV radiation, and a possiblegrating or diffractive structure, is substantially transmissive for theEUV radiation of the beam of radiation PB. However, due to thedifference in indices of refraction when the EUV radiation enters andleaves the protrusion, there may be some diffractive losses of the EUVradiation of the beam of radiation PB.

In another variation, the protrusions are arranged in such a way that apart of EUV radiation of a beam of radiation including EUV radiation,having an angle of incidence between 0° and 90°, only passes oneprotrusion, as shown in FIG. 9, or substantially every part of the beamof radiation PB passes only one protrusion.

The profile (which may be diffractive and optimised at, for example,about 13.5 nm or other EUV wavelengths) is configured such that thedesired radiation included in radiation beam PB only passes one period pof the profile and is reflected at the mirror or mirror surface 300 atposition 305. The angle of incidence of the beam of radiation PB withmirror 300 is indicated with reference α, whereas the angle of incidenceof the beam of radiation PB with a side surface of a protrusion is shownas α′₁. The angle of incidence α′₁ may remain small to minimisereflection of the beam of radiation PB. Also, the upper surface of theprotrusions of the profile can be angled, as in a blazed grating, sothat the reflection of the undesired radiation (e.g. with wavelengthshigher than EUV, e.g. UV VIS and IR) on the protrusion surface isdirected in a different direction than the desired radiation. The length380 and height h of the protrusions, the period p in which theprotrusions are arranged, as well as the angle of incidence α, may bechosen such that the beam of radiation PB only passes one protrusion.

The profile on the mirror can be produced by ruling (scratching a line)with a diamond tool. It is also possible to produce the profile by ionetching of a sinusoidal structure. Producing a block profile on agrazing incidence-mirror using lithographic techniques, with subsequention etching is also possible. The profile as shown in FIG. 9 is given asan example; other profiles are also possible, provided that EUVradiation is substantially not absorbed (i.e. is substantiallytransmissive), and diffraction of EUV radiation is small (e.g. less thanabout 30% is diffracted). FIG. 9 shows an example of a laminar squarewave profile. However, this embodiment also includes the configurationwherein a laminar sawtooth profile is used, or wherein a periodicallystructured sawtooth profile or periodically structured square waveprofile is used.

In case a laminar sawtooth profile is used, the length 380 and height hof the protrusions, the period p in which the protrusions are arranged,and the angle ba, as well as the angle of incidence a may be chosen suchthat the beam of radiation PB only passes one protrusion.

Similarly this applies to the periodically structured profiles, howeverin such embodiment the period p includes two periods, since a twodimensional structure is formed.

This embodiment may also include a profile wherein the protrusions havea length 380 that is smaller than half a period p. This is shown in FIG.9 for the laminar arranged square wave profile, but this may also applyfor a sawtooth profile and for two dimensional profiles.

In the embodiments above, a lithographic apparatus was described ingeneral, and mirrors including one or more protrusions including amaterial selected from Be, B, C, Si, P, S, K, Ca, Sc, Br, Rb, Sr, Y, Zr,Nb, Mo, Ru, Rh, Ag, Ba, La, Ce, Pr, Pa or U were described. A mirror asdescribed above may include at least two different protrusions includingtwo or more different materials. This will be further explained withreference to FIGS. 10 and 11.

Referring to FIG. 10, a mirror 300 includes one or more firstprotrusions LP1 a including a first material m1 selected from at leastone of Be, B, C, P, S, K, Ca, Sc, Br, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Ag,Ba, La, Ce, Pr, Pa or U (protrusion LP1 a), and one or more secondprotrusions LP1 b including a second material m2 selected from at leastone of Be, B, C, Si, P, S, K, Ca, Sc, Br, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh,Ag, Ba, La, Ce, Pr, Pa or U (protrusion LP1 b). The first and secondmaterials may not be the same, for example Si and Mo, Mo and Zr, C andSi, Be and Zr, etc. In another example, m1 is Be or Zr, and m2 is Si₃N₄or SiO₂. More materials than m1 and m2, and/or more protrusions than thefirst and second protrusions may be applied on mirror 300. In FIG. 10, alaminar sawtooth profile on mirror 300 is shown. In contrast to thesawtooth profiles shown in FIG. 7, the laminar sawtooth profile hereincludes structures of two materials. The structure has a height h1, andincludes the first protrusion LP1 a of a first material m1, with a blazeangle ba2 and a height h1, and a second protrusion LP1 b of a secondmaterial m2 with a height h2 and with a blaze angle ba3. Theseprotrusions (of two or more materials) form together one protrusion orstructure with a blaze angle ba. The protrusions may form a regularstructure with period p.

The protrusions on mirror 300, as shown in FIG. 10 may form a gratingthat may be blazed at EUV wavelengths, but in another variation of thisembodiment they may also be blazed at other wavelengths, e.g. UV, VIS orIR wavelengths.

The blaze angle ba can be chosen in such a way that undesired radiationis deflected from the direction of the desired radiation, see above. Theblaze angle ba depends on angle ba2 of material m1 of the protrusion LP1a (first protrusions), angle ba3 of material m2 of the protrusion LP1 b(second protrusion), and of heights h1 and h2 of the protrusions LP1 aand LP1 b, which are in this case on top of each other.

As an alternative to the blazed profile, a laminar profile of the typeshown in FIG. 11 may be used. As shown in FIG. 11, the laminar profilehas a square wave surface profile with period p, which may be constant.The protrusions are located next to another in an alternating sequenceand have different heights, i.e. protrusion LP1 a of material m1 hasheight h1 and protrusion LP1 b of material m2 has height h2. The lengthsof the protrusions are shown in this figure by reference 380 a forprotrusion LP1 a (first protrusion) of material m1 and by reference 380b for protrusion LP1 b (second protrusion) of material m2. In FIG. 11,lengths 380 a and 380 b are the same and the length of both when addedis equal to period p.

By selecting the materials based on their indices of refraction, heightsh1 and h2, and when applicable lengths 380 a and 380 b or the angles ba,ba2 and ba3, distance (period p) of the first and second protrusions,and angle of incidence of the beam of radiation on the mirror, a mirrorcan be obtained that reflects at a certain angle the desired radiationwhile correcting by the different materials the optical path lengthdifference, such that it does not vary over the minoring surface,whereas light of undesired wavelengths is absorbed and/or deflected atother angles.

By way of example, this is shown in FIG. 11. Ray r1 of beam of radiationPB enters the first protrusion LP1 a of material m1 and maysubstantially be transmitted through this protrusion and reflected atthe mirror or mirror surface 300 on position 305. After being reflected,ray r1 leaves the protrusion. Ray r2, also included in beam of radiationPB, enters the second protrusion LP1 b of material m2 and is reflectedat position 306 at the minor or on the mirror surface 300. Without thepresence of this second protrusion LP1 b, the optical path length(through the first and second protrusion and vacuum) is different fordifferent rays. To compensate for this difference in optical pathlength, the second protrusion LP of material m2 is present. Thismaterial m2 has another index of refraction. By choosing the appropriatedimensions of the protrusions and an angle of incidence, as mentionedabove, the second protrusion LP1 b of material m2 can compensate for theoptical path difference for the different rays in the beam of radiationPB, thereby providing an optical path length difference that does notvary over the mirror surface. This means that the optical path lengthdifference is zero or an integer times the wavelength. The vacuum abovethe protrusions should also be taken into account for the determinationof the optical path length difference. In this way, the optical pathlengths, i.e. path length times the indices of refraction are for raysr1 and r2, starting from wave front WF_(b), before entering protrusionsLP1 a and LP1 b, to wave front WF_(a), after the protrusions LP1 a andLP1 b, are the same or the difference between the optical path lengthsfor rays r1 and r2 is an integer times the wavelength (e.g. 13.5 nmradiation). Hence, in this embodiment an optical path length differenceis created for EUV radiation that does not vary over the mirror surface.

For Si, having an index of refraction that is near unity for EUVradiation, the presence of such compensating second protrusions might beless necessary. However, when applying one of the other materials, thepresence of the second protrusion LP1 b might be desirable.Nevertheless, Si may also create a small optical pathlength differencebetween the different rays, as shown for r1 and r2 in FIG. 11, within abeam of radiation PB, that may be compensated for by the presence ofsecond protrusions LP1 b of Be, B, C, P, S, K, Ca, Sc, Br, Rb, Sr, Y,Zr, Nb, Mo, Ru, Rh, Ag, Ba, La, Ce, Pr, Pa or U

This embodiment also includes a mirror having a mirror surface, forexample in a lithographic apparatus, wherein the protrusions arearranged in such a way that a part of EUV radiation of a beam ofradiation including EUV radiation, having an angle of incidence between0° and 90°, only passes in one period p one first protrusion LP1 a ofmaterial m1 and one second protrusion LP1 b of material m2. This meansthat the length of the protrusions 380 a and 380 b is smaller than theperiod p.

This embodiment also includes a mirror wherein the profile ofprotrusions forms a grating, that is blazed (sawtooth) or optimised(square wave) at the desired radiation, e.g. 13.5 nm or anotherwavelength. Profiles of these types on mirrors may have a onedimensional or two dimensional character. Since these protrusions aretransparent to EUV radiation, the desired wavelengths are reflected bythe mirror 300, and the undesired wavelengths of the radiation areabsorbed, refracted and/or deflected.

In general, this embodiment describes a mirror, having at least onemirror surface 300 that includes one or more first protrusions LP1 aincluding a first material m1 and one or more second protrusions LP1 bincluding a second material m2, and the first and second materials arenot the same. Such a mirror 300 can be used as an optical filter, whenthe materials that are used are transparent for the desired wavelength,or optimised to be transparent for the desired wavelengths. Otherwavelengths, not desired wavelengths, may be absorbed by the materials,and when a grating for a certain wavelength is chosen (grating or blazedgrating), the undesired wavelengths might also be partially deflected.

This embodiment also includes a lithographic apparatus, for example asdescribed above, including the mirror as described in this embodiment.

For providing the above described structures, the blazed structure maybe provided by ruling a layer of material m1 with a diamond tool,depositing material m2, and ruling with a diamond tool for a secondtime. For the square shaped structures, a lithographic process may beused.

According to another embodiment, the protrusions 301 substantiallyinclude Si and the transmissive top layer TL with structure RSsubstantially includes Ru.

According to another embodiment, an optical element, which may be amirror, for example a grazing incidence mirror, normal incidence mirror,multilayer mirror, is provided with a protective coating. The protectivecoating includes a transmissive top layer TL including structure RSaccording to the present invention. A protective coating is providedwith enhanced transmission, with respect to a protective layer withoutsuch structure RS.

In the table below, rms values for structures of the transmissive toplayer are given for different wavelengths: 5, 13.5 and 20 nm. Thetransmissive top layer with these rms roughness values may be present onthe optical elements of the present invention, as described above in anumber of embodiments. Depending upon the application and the methodsused to provide to transmissive top layer and its structure, it shouldbe appreciated that rms roughnesses in the ranges as described below maybe chosen.

Further in the table, a variation on this embodiment is provided. Forexample, using a wavelength of about 13.5 nm, one may provide an rmsroughness equal to or larger than about 1.4 nm and equal to or smallerthan about 54 nm for spatial periods equal to or larger than about 2.7nm and equal to and smaller than about 6.75 nm. It should be appreciatedthat smaller rms roughnesses than 54 nm may be chosen, for example anrms roughness equal to or larger than about 2.7 nm and equal to orsmaller than about 27 nm for spatial periods equal to or larger thanabout 2.7 nm and equal to and smaller than about 6.75 nm may beprovided.

TABLE 3 rms roughness values for 5, 13.5 and 20 nm for different spatialperiods according to the present invention: λ 5.0 nm Spatial periodrange (nm) equal to or 1.0 — equal to or 2.5 larger than smaller thanRMS (nm) equal to or 0.5 — equal to or 20.0 larger than smaller than RMS(nm) (variation) equal to or 1.0 — equal to or 10.0 larger than smallerthan Spatial period range (nm) larger than 2.5 — equal to or 1 musmaller than RMS (nm) equal to or 0.1 — equal to or 5.0 larger thansmaller than RMS (nm) (variation) equal to or 0.1 — equal to or 2.5larger than smaller than λ 13.5 nm Spatial period range (nm) equal to or2.7 — equal to or 6.75 larger than smaller than RMS (nm) equal to or 1.4— equal to or 54.0 larger than smaller than RMS (nm) (variation) equalto or 2.7 — equal to or 27.0 larger than smaller than Spatial periodrange (nm) larger than 6.75 — equal to or 1 mu smaller than RMS (nm)equal to or 0.27 — equal to or 13.5 larger than smaller than RMS (nm)(variation) equal to or 0.27 — equal to or 6.75 larger than smaller thanλ 20.0 nm Spatial period range (nm) equal to or 4.0 — equal to or 10.0larger than smaller than RMS (nm) equal to or 2.0 — equal to or 80.0larger than smaller than RMS (nm) (variation) equal to or 4.0 — equal toor 40.0 larger than smaller than Spatial period range (nm) larger than10.0 — equal to or 1 mu smaller than RMS (nm) equal to or 0.4 — equal toor 20.0 larger than smaller than RMS (nm) (variation) equal to or 0.4 —equal to or 10.0 larger than smaller than

For spatial periods larger than about 6.75 nm and equal or smaller thanabout 1 μm, the rms roughness is in this embodiment equal to or largerthan about 0.1 nm and equal to or smaller than about 5.0 nm. In avariation, an rms roughness equal to or larger than about 0.1 nm andequal to or smaller than about 2.5 nm for spatial periods larger thanabout 6.75 nm and equal or smaller than about 1 μm may be provided.

While specific embodiments of the present invention have been describedabove, it will be appreciated that the invention may be practicedotherwise than as described. The description is not intended to limitthe invention. Further, some of the embodiments especially describe EUVapplications and EUV optical elements. However, the present inventioncan also be applied to optical elements for other spectral ranges, forexample. UV or VIS. Further, the drawings are illustrative and providedfor an understanding of the present invention. The invention is notlimited to those elements shown in the schematic drawings. For example,more layers than shown may be present.

What is claimed is:
 1. A lithographic apparatus, comprising: anillumination system configured to condition a beam of radiation; asupport configured to support a patterning device, the patterning deviceconfigured to impart the beam of radiation with a pattern in its crosssection; a substrate table configured to hold a substrate; and aprojection system configured to project the beam of radiation after ithas been patterned onto a target portion of the substrate, wherein atleast one of the illumination system and the projection system includesan optical element, the optical element comprising a layer at leastpartially transmissive for EUV radiation with a wavelength λ in therange of 5-20 nm, and a top layer transmissive for EUV radiation withthe wavelength λ in the range of 5-20 nm, the top layer including astructure having an rms roughness value equal to or larger than λ/10 forspatial periods equal to or smaller than λ/2.
 2. An apparatus accordingto claim 1, wherein the rms roughness is equal to or smaller than λ forspatial periods larger than λ/2 and equal to or smaller than 1 μm.
 3. Anapparatus according to claim 1, wherein the at least partiallytarnsmissive layer includes a surface with a structure with an rmsroughness value equal to or larger than λ/50 and equal to or smallerthan 4*λ for spatial periods equal to or smaller than λ/2.
 4. Anapparatus according to claim 1, wherein the top layer includes amaterial selected from Be, B, C, Si, P, S, K, Ca, Sc, Br, Rb, Sr, Y, Zr,Nb, Mo, Ru, Rh, Ag, Ba, La, Ce, Pr, Ir, Au, Pa or U and combinationsthereof.
 5. An apparatus according to claim 1, wherein the top layer andthe structure include Ru.
 6. An apparatus according claim 1, wherein theoptical element includes a mirror having a mirror surface, the mirrorsurface includes a protrusion transmissive for EUV radiation with thewavelength λ in the range of 5-20 nm, and at least part of the mirrorsurface further includes the top layer including the structure.
 7. Anapparatus according to claim 1, wherein the optical element furthercomprises a profile having height differences, thereby providingcavities and elevations having a predetermined maximum heightdifference; and a substantially transmissive layer in the cavities andon the elevations of the optical element, wherein the substantiallytransmissive layer has a planar surface on which the top layer andstructure are provided.
 8. An apparatus according to claim 1, whereinthe rms roughness value of the top layer is 2 nm or larger for spatialperiods equal to or smaller than λ/2.
 9. An apparatus according to claim1, wherein the optical element includes optical filters, opticalgratings, mirrors or lenses.
 10. An apparatus according to claim 1,wherein the optical element includes a mirror with a mirror surface anda tilted multilayer stack which is tilted with respect to the mirrorsurface.
 11. A device manufacturing method, comprising: providing a beamof radiation; patterning the beam of radiation with a pattern in itscross-section; and projecting the patterned beam of radiation onto atarget portion of a substrate, wherein the beam of radiation is incidenton an optical element including a top layer transmissive for EUVradiation with a wavelength λ in the range of 5-20 nm, the top layerincluding a structure having an rms roughness value equal to or largerthan λ/10 for spatial periods equal to or smaller than λ/2.
 12. A devicemanufactured according to the method of claim 11.